shRNA suppression of LRP1 and megakaryocyte colony formation. (A) Representative flow cytometry of 3T3 cells (positive control for LRP1) stably transfected with different shRNA viral vectors. The solid gray line represents cells that were unstained. The broken gray line is LRP1 expression on cells expressing the empty lentiviral vector. Solid black line shows the decrease in surface LRP1 expression after stable transfection with the LRP1 shRNA virus. (B) Same as panel A except for murine bone marrow cells after culture in media containing TPO and puromycin for 5 days. The solid gray line represents isotype control. The broken gray line is cells transfected with the negative viral vector. The solid dark line represents cells transfected with LRP1 shRNA. (C) Quantitation of change in mean fluorescence index (MFI) in murine bone marrow cells transfected with virus. Data represent mean +1 SD for 3 independent experiments. Viral titers were between 1 to 2 × 10¹¹ viral particles/mL. (D) Effect of LRP1 shRNA on megakaryopoiesis (meg) using mPF4^−/− bone marrow and hPF4^High bone marrow expressed relative to megakaryopoiesis with the control empty vector. ■ is relative level of megakaryocyte seen after transfection with the empty lentiviral vector, and □ is relative level after transfection with the LRP1 shRNA vector. Data represent mean +1 SD for 4 experiments, each performed in duplicate. *P < .006 for LRP1 versus negative control for hPF4. shRNA had no effect on colony formation in mPF4^−/− bone marrow. (E) Effect of LRP1 shRNA megakaryocyte colony formation in mPF4^−/− bone marrow treated with exogenous PF4 (25 μg/mL). Percentage of meg colonies were normalized as in panel D. Ctl indicates control studies with empty vector. Data represent mean +1 SD. Numbers in bars represent times experiments were performed (each in duplicate). *P < .008 for LRP1 versus empty virus in the presence of PF4.